improve background query handling (#38)

# improve background query handling

## ♻️ Current situation & Problem
HealthKit sometimes sends a background query notification even though no
new samples were added to the database (i've seen this happen in e.g.
the simulator, my guess is that in that case even though there simply
aren't any samples it for some reason still wants to send one initial
query update).
This interferes a bit with our existing sample type correctness checking
(there is a check that the array of new samples contains at least one
sample with the queried-for sample type, which is of course false in
this case).
This PR reworks this a bit, essentially simply adding a check to return
early if the array is empty.

## ⚙️ Release Notes 
- improved handling of background queries to avoid unnecessary error
messages printed to stdout

## 📚 Documentation
n/a

## ✅ Testing
n/a

## 📝 Code of Conduct & Contributing Guidelines 

By submitting creating this pull request, you agree to follow our [Code
of
Conduct](https://github.com/StanfordSpezi/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/StanfordSpezi/.github/blob/main/CONTRIBUTING.md):
- [x] I agree to follow the [Code of
Conduct](https://github.com/StanfordSpezi/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/StanfordSpezi/.github/blob/main/CONTRIBUTING.md).

---------

Signed-off-by: Lukas Kollmer <hey@lukaskollmer.de>

Sources/SpeziHealthKit/Health Data Collection/HealthKitSampleCollector.swift

@@ -91,22 +91,29 @@ final class HealthKitSampleCollector<Sample: _HKSampleWithSampleType>: HealthDat
                         // if the sample collector has been turned off, we don't want to process these.
                         return
                     }
-                    guard case let .success((sampleTypes, completionHandler)) = result else {
-                        return
-                    }
-                    guard sampleTypes.contains(self.sampleType.hkSampleType) else {
-                        self.healthKit.logger.warning("Received Observation query types (\(sampleTypes)) are not corresponding to the CollectSample type \(self.sampleType.hkSampleType)")
-                        completionHandler()
-                        return
-                    }
-                    do {
-                        try await self.anchoredSingleObjectQuery()
-                        self.healthKit.logger.debug("Successfully processed background update for \(self.sampleType.hkSampleType)")
-                    } catch {
-                        self.healthKit.logger.error("Could not query samples in a background update for \(self.sampleType.hkSampleType): \(error)")
+                    switch result {
+                    case .failure(let error):
+                        self.healthKit.logger.error("Error in background delivery: \(error)")
+                    case let .success((sampleTypes, completionHandler)):
+                        defer {
+                            // Inform to HealthKit that the data has been processed:
+                            // https://developer.apple.com/documentation/healthkit/hkobserverquerycompletionhandler
+                            completionHandler()
+                        }
+                        guard !sampleTypes.isEmpty else {
+                            return
+                        }
+                        guard sampleTypes.contains(self.sampleType.hkSampleType) else {
+                            self.healthKit.logger.warning("Received Observation query types (\(sampleTypes)) are not corresponding to the CollectSample type \(self.sampleType.hkSampleType)")
+                            return
+                        }
+                        do {
+                            try await self.anchoredSingleObjectQuery()
+                            self.healthKit.logger.debug("Successfully processed background update for \(self.sampleType.hkSampleType)")
+                        } catch {
+                            self.healthKit.logger.error("Could not query samples in a background update for \(self.sampleType.hkSampleType): \(error)")
+                        }
                     }
-                    // Provide feedback to HealthKit that the data has been processed: https://developer.apple.com/documentation/healthkit/hkobserverquerycompletionhandler
-                    completionHandler()
                 }
                 isActive = true
                 queryVariant = .backgroundDelivery(queryInvalidator)

Tests/UITests/TestApp.xctestplan

@@ -9,6 +9,20 @@
     }
   ],
   "defaultOptions" : {
+    "codeCoverage" : {
+      "targets" : [
+        {
+          "containerPath" : "container:..\/..",
+          "identifier" : "SpeziHealthKit",
+          "name" : "SpeziHealthKit"
+        },
+        {
+          "containerPath" : "container:..\/..",
+          "identifier" : "SpeziHealthKitUI",
+          "name" : "SpeziHealthKitUI"
+        }
+      ]
+    },
     "commandLineArgumentEntries" : [
       {
         "argument" : "-AppleLocale",